1. Field of the Invention
This invention relates to a gas-introduced injection molding and a method of obtaining the same, and it is applicable to the manufacture of resin products, of which the appearance and quality are important, such as automobile interior and exterior fittings, for instance bumpers, dashboards, wheel caps, door panels, etc., and also household electric product casings, television receiver cabinets, etc.
2. Description of the Related Art
The gas-introduced injection molding process is well known in the art as a method of injection molding to obtain moldings which are light in weight and highly rigid.
In this method, the injection molding is carried out while introducing an inert gas such as nitrogen gas under high pressure into molten resin filling a die cavity. In this case, with inflating force of the introduced high pressure gas it is possible to obtain a satisfactory molding product with the surface thereof free from sink mark.
There is also a gas-introduced injection molding process of developing type, in which molten resin is charged into a die, which is closed imperfectly such that the charged molten resin does not leak to the outside, and the charged molten resin is developed with the closing of the die so that the entire die cavity is filled with the molten resin.
With this process, it is possible to reliably prevent defects due to short shots because the entire die cavity can be perfectly filled with molten resin with the spread thereof caused when the die is closed.
In the prior art, however, if it is intended to produce a gas-introduced injection molding product having a reduced thickness, the thickness of resin enclosing the inner space is also reduced, thus making it impossible to obtain sufficient rigidity.
Therefore, the process is not applicable to products, which have a sheet-like portion and nevertheless is required to have a predetermined rigidity, such as automobile bumpers and household electric product casings.
Meanwhile, when the prior art gas-introduced injection molding process is applied to the production of a small thickness molding product, sufficient pushing force fails to be provided near the edges of the die cavity even during the charging of molten resin because of high resistance offered to the flow of the molten resin in the die cavity. Therefore, the leading end of the flow of molten resin is liable to be stopped tentatively.
If this occurs, by causing the molten resin to begin to start again by closing the die or by introducing gas a trace of re-flow (hereinafter referred as hesitation mark) is generated in the surface of the molding product to deteriorate the quality thereof.
An object of the invention is to provide a high quality gas-introduced injection molding, which can ensure sufficient rigidity even if it is then and is free from hesitation marks or like sink mark, and also a method of obtaining the same molding.